Battery-operated electrical devices, for example, small electrical devices such as electric toothbrushes or electric shavers, are normally charged in a charging station that is external to the device. During the charging process, it is then possible to initiate certain functions, such as, for example, a charge control display, and to suppress certain functions, like turning on the toothbrush. Therefore, there is a requirement to be able to detect at any point in time whether the device is in the charging station or not. To this end a more or less complex logic unit is provided in the device, which evaluates the presence of a charging voltage for the battery. This logic unit may be, but does not have to be, separate from the circuit components (charging current circuit and control logic) that control the charging process itself. Because the charging devices normally resemble an ideal current source rather than an ideal voltage source, the charging voltage clearly drops below the no-charge value when under a charge—that is, when charging current is flowing. This charging voltage is then barely enough to be reliably evaluated by a logic unit, especially in devices that have small battery cell voltages.
It is known to use the charging current circuit on the source side of the charging current switch via an ohm resistive potentiometer to tap the charging voltage, which is variable as a function of the charging current flux. If a charging voltage is present, that is, if the charging current circuit is open, the charging voltage is at its maximum, and if a charging current is flowing the charging voltage drops. A microcontroller performs the evaluation. It is disadvantageous that current flows via the ohm resistive potentiometer continuously, which leads to continuous energy consumption. Thus, it would be desirable to provide a circuit arrangement that makes it possible to detect a charge without great complexity.